This invention relates to a pneumatic servomotor used for assisting in the braking of motor vehicles.
More particularly, the present invention relates to a pneumatic servomotor for an assisted braking, comprising: a rigid casing; a moving airtight partition wall, defining a front chamber and a rear chamber inside the casing, the front chamber being operatively under a first pressure and the rear chamber being selectively connected to the first chamber or under a second pressure higher than the first one; a pneumatic piston travelling with the moving partition wall; a control rod, moving inside the piston as a function of an input force, selectively exerted in an axial actuation direction towards the front chamber, and of a return force, applied by a main spring in an axial return direction, reverse to the axial actuation direction, the return force biasing the control rod towards a rest position, while the input force biases the control rod towards either an intermediate actuation position or an end actuation position, depending on whether the input force is applied at a speed which is lower or higher than a given limit speed a plunger, housed within the piston and driven by the control rod; a three-way valve, including an annular seat borne by a rear section of the plunger, such valve connecting the rear chamber with the front chamber when the control rod is in the rest position, and subjecting the rear chamber to the second pressure when the control rod is in one of its actuation positions; force-transmitting means, adapted for receiving and transmitting at least one assistance force, exerted by a front face of the piston when the valve puts the rear chamber under the second pressure, the plunger including a front section comprising retaining means, intended to tie down the front section of the plunger to the pneumatic piston.
Such devices are well-known from the prior art, as disclosed, for instance, in patent specifications U.S. Pat. No. 3,470,697, FR-A-2 532 084 and FR-A-2 658 466.
Recent investigations revealed that quite a number of drivers, when confronted with an emergency braking situation, underestimated the risks actually incurred and, after having jammed the brakes on, would somewhat release the braking force at the very time when a considerable force should have been maintained in order to avoid an accident.
This established fact led to various solutions aiming at correcting possible behaviour lapses of a driver, whether inexperienced or panic-stricken.
Therefore, it is the object of the present invention to provide a simple solution to the above-mentioned problem regarding the maintenance of a high braking force, following the jamming on of the brakes.
This object is achieved in that the servomotor according to the invention, as defined in the above introductory section, is characterised in that the retaining means comprise a unidirectional clutch device for the driving of the force-transmitting means, independently of the control rod and of the rear section of the plunger when they are biased by the pneumatic piston, as a result of the actuation of the control rod, the speed of which is higher than a predetermined value.
In accordance with a feature of the present invention, the unidirectional clutch device comprises a cylindrical sleeve, capable of freely sliding around a front part of the rear section of the plunger, and a locking key including a central opening surrounding the sleeve and having an inner diameter slightly greater than the outer diameter of the cylindrical sleeve, and provided with a first and a second indexing parts, diametrically opposed in relation to the central opening.
At rest, and under the action of a resilient means, the locking key advantageously bears on a stationary part of the servomotor through the first indexing part, and on the pneumatic piston through the second indexing part.
Preferably, at rest, the sleeve is biased backwards into abutment against a shoulder on the rear section of the plunger, via a resilient means.
The front section of the plunger may comprise a finger, slidingly fitted on the front end of the rear section of the plunger, the finger being capable of contacting the reaction disk when the sleeve is biased by the locking key, as a result of the actuation of the control rod, the speed of which is higher than a predetermined value.
The rear rest position of the plunger may advantageously be defined by the bearing on the locking key, under the action of the main spring, of a stop member situated on the rear section of the plunger.
In that case, the rear section of the plunger may consist of a pin, perpendicular to the plunger axis and passing through elongated openings made in the sleeve.
The stop member, located on the rear section of the plunger, may also be constituted by a shoulder, cooperating with at least one part of the locking key, traversing a slot provided in the sleeve.
According to another embodiment, at rest, the first indexing part of the locking key bears on an intermediate element, bearing itself on a stationary part of the servomotor.
The unidirectional clutch device then may advantageously be contained in a cartridge, constituting the front face of the pneumatic piston, the intermediate element protruding rearward from the cartridge.